I have a project that I wish to be released open source.
the problem is that it includes some marketplace content, so if I released it would be like releasing also the marketplace contents for free.
is it ok to do that?
You can’t distribute Marketplace assets “uncooked”, unfortunately. 
(Also a problem for releasing UE4 games that can be modded).
They have to be packaged/compiled into the game. This won’t stop you releasing the other code for your own Blueprints/C++ parts of the game. (EDIT: Sorry, I meant that you could release the other parts of your code freely, like ARK have, but even then, it’s not Open Source, as the Engine is freely available, but not Open Source.)
so open source projects cannot really be made 
No. Even without the Marketplace bits, the Unreal Engine code itself isn’t Open Source, so you can’t do it at all.
really? I can’t create a public repository on github and place my own project there?
Yes, you can. Open Source has a very specific meaning, though.
To confuse you further (:)) The Unreal Engine’s source is open, but not Open Source. (It’s freely available, you can change what you want and you can sell games for money, but it never stops belonging to Epic.)
let’s recap.
Can I make my own project and place it in a public repository on github?
can I add marketplace contents to that project?
Oh, I see what you are getting at. You don’t want to make your project Open Source, you just want to store it, even if it contains Marketplace assets, in a publicly accessible place. Is that right?
You wouldn’t back up your whole project on a non-LFS (and expensive) Github anyway, just the Blueprints/C++ sections you create in Game/Content. Github isn’t great at backing up stuff like binary files (for images etc). Therefore you wouldn’t put Marketplace environment, mesh, animation, etc files on there.
But what about Marketplace Blueprints?
Although it is physically possible to put them on Github, I wouldn’t. Anyway, your assets from the Marketplace section are backed up in your vault, and by Epic, so you don’t have to put those online.
I’m not aware of any explicit warning not to do this, though. This needs checking.
EDIT: Have a look at this.
I still don’t get it, let’s not use the term “open source” that is confusing.
I want a public repository where everyone can join and branch my project if they want.
I also want to use some marketplace contents.
how do I achieve that?
To be honest, you are probably better speaking to the organ-grinder (Epic’s legal department) rather than the monkey* (me). Raise a question on AnswerHub, and they will let you know. I think that the question you want answered is you want to host Marketplace content on Github, yes?
- I wonder what the non-English users will make of that metaphor.
I also want to use Marketplaces items in open project (with custom license).
One possible solution - release only part of sources and replace Marketplace items by dummy objects…
This is some information abou marketplace content license (even for free items from Epic) - https://answers.unrealengine.com/questions/348037/is-it-oklegal-to-publish-my-game-on-github.html
“The Product may not contain any Engine Tools or any Marketplace Content Distributed in uncooked source format.”
I would think no, you would only be able to include your own custom files, and not stuff that you purchased from the marketplace.
Ouch, not possible then
thanks everyone
But if you want to do some modding support… You could try replace marketplace content by dummies in sources or get permission directly from author…
Nope, I just want an open source project, where everyone can contribute or get the latest version of the game and modify it.
Unless there is a way to add some “cooked” contents within the editor itself I don’t think I can do it
Your contributors would be required to sign up for an Epic account and abide by Epic’s licensing restrictions, but once they cross that hurdle, there is no problem.
This would be a violation of the Marketplace license, but there is nothing stopping you from contacting someone who has assets on the Marketplace and purchasing a different license through some other sort of transaction.
How can I sure about this? Is it enough just to ask them or they should sign some agreement with me? Or how it could be done?